Hi Robert,
Unfortunately there's not much available to stop threading at the moment - you are at the mercy of the sending party to create a brand new email every time to guarantee a new interaction.
Most of our issues occur from customers using a reply/forward of an email they've previously written as a sudo template (i.e. update some of the body content and subject, remove the Re: or Fw:, then send the email) - we often get emails with 10-20 threads happening on a single interaction - which skews reporting, forecasting and blows out handling time.
The only currently technical option that slightly helps is the new(ish) "Route Email to multiple destinations" org setting and turning this on. At least emails with multiple email addresses for different queues (i.e. one for sales, one for support) will now generate separate interactions (rather than choosing only one queue/flow at random and threading the rest).

There are ideas to be able to control threading (see https://genesyscloud.ideas.aha.io/ideas/DIG-I-4) that's in development - but currently no details on exactly how it'll work or whether it'll solve our problems.
Hope that helps somewhat.
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Jeff
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